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Ohm’s Law Mitsuko J. Osugi Physics 409D Winter 2004 UBC Physics Outreach
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Ohm’s Law <ul><li>Current through an ideal conductor is proportional to the applied voltage </li></ul><ul><ul><li>Conductor is also known as a resistor </li></ul></ul><ul><ul><li>An ideal conductor is a material whose resistance does not change with temperature </li></ul></ul><ul><li>For an ohmic device, </li></ul><ul><ul><li>V = Voltage (Volts = V) </li></ul></ul><ul><ul><li>I = Current (Amperes = A) </li></ul></ul><ul><ul><li>R = Resistance (Ohms = Ω ) </li></ul></ul>
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Current and Voltage Defined <ul><li>Conventional Current : (the current in electrical circuits) </li></ul><ul><li>Flow of current from positive terminal to the negative terminal. </li></ul><ul><li>- has units of Amperes (A) and is measured using ammeters . </li></ul><ul><li>Voltage : </li></ul><ul><li>Energy required to move a charge from one point to another. </li></ul><ul><li>- has units of Volts (V) and is measured using voltmeters . </li></ul>Think of voltage as what pushes the electrons along in the circuit, and current as a group of electrons that are constantly trying to reach a state of equilibrium .
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Ohmic Resistors <ul><li>Metals obey Ohm’s Law linearly so long as their temperature is held constant </li></ul><ul><ul><li>Their resistance values do not fluctuate with temperature </li></ul></ul><ul><ul><ul><li>i.e. the resistance for each resistor is a constant </li></ul></ul></ul><ul><li>Most ohmic resistors will behave non-linearly outside of a given range of temperature, pressure, etc. </li></ul>
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Voltage and Current Relationship for Linear Resistors Voltage and current are linear when resistance is held constant.
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Ohm’s Law continued <ul><li>The total resistance of a circuit is dependant on the number of resistors in the circuit and their configuration </li></ul><ul><ul><li>Series Circuit </li></ul></ul><ul><ul><li>Parallel Circuit </li></ul></ul>
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Kirchhoff’s Current Law <ul><li>Current into junction = Current leaving junction </li></ul>The amount of current that enters a junction is equivalent to the amount of current that leaves the junction
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Kirchhoff’s Voltage Law <ul><li>Net Voltage for a circuit = 0 </li></ul>Sum of all voltage rises and voltage drops in a circuit (a closed loop) equals zero
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Series Circuit <ul><li>Current is constant </li></ul><ul><li>Why? </li></ul><ul><ul><li>Only one path for the current to take </li></ul></ul>
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<ul><li>We’ve now looked at how basic electrical circuits work with resistors that obey Ohm’s Law linearly. </li></ul><ul><li>We understand quantitatively how these resistors work using the relationship V=IR, but lets see qualitatively using light bulbs. </li></ul>
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The Light Bulb and its Components <ul><li>Has two metal contacts at the base which connect to the ends of an electrical circuit </li></ul><ul><li>The metal contacts are attached to two stiff wires, which are attached to a thin metal filament. </li></ul><ul><li>The filament is in the middle of the bulb, held up by a glass mount. </li></ul><ul><li>The wires and the filament are housed in a glass bulb, which is filled with an inert gas, such as argon. </li></ul>
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Light bulbs and Power <ul><li>Power dissipated by a bulb relates to the brightness of the bulb. </li></ul><ul><li>The higher the power, the brighter the bulb. </li></ul><ul><li>Power is measured in Watts [W] </li></ul><ul><li>For example, think of the bulbs you use at home. The 100W bulbs are brighter than the 50W bulbs. </li></ul>
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Bulbs in series experiment <ul><li>One bulb connected to the batteries. Add another bulb to the circuit in series . </li></ul><ul><li>Q: When the second bulb is added, will the bulbs become brighter, dimmer, or not change? </li></ul><ul><li>We can use Ohm’s Law to approximate what will happen in the circuit in theory: </li></ul>
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Bulbs in parallel experiment <ul><li>One bulb connected to the batteries. Add a second bulb to the circuit in parallel . </li></ul><ul><li>Q: What happens when the second bulb is added? </li></ul><ul><li> We can use Ohm’s Law to approximate what will happen in the circuit: </li></ul>
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Light bulbs are not linear <ul><li>The resistance of light bulbs increases with temperature </li></ul>The filaments of light bulbs are made of Tungsten, which is a very good conductor. It heats up easily.
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As light bulbs warm up, their resistance increases. If the current through them remains constant: <ul><li>They glow slightly dimmer when first plugged in. </li></ul><ul><ul><li>Why? </li></ul></ul><ul><ul><li>The bulbs are cooler when first plugged in so their resistance is lower. As they heat up their resistance increases but I remains constant P increases </li></ul></ul><ul><li>Most ohmic resistors will behave non-linearly outside of a given range of temperature, pressure, etc. </li></ul>
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Voltage versus Current for Constant Resistance The light bulb does not have a linear relationship. The resistance of the bulb increases as the temperature of the bulb increases.
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“ Memory Bulbs” Experiment <ul><li>Touch each bulb in succession with the wire, each time completing the series circuit </li></ul><ul><li>Q: What is going to happen? </li></ul><ul><li>Pay close attention to what happens to each of the bulbs as I close each circuit. </li></ul>
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“ Memory Bulbs” Continued… <ul><li>Filaments stay hot after having been turned off </li></ul><ul><li>In series, current through each resistor is constant </li></ul><ul><ul><li>smallest resistor (coolest bulb) has least power dissipation, therefore it is the dimmest bulb </li></ul></ul>How did THAT happen?? Temperature of bulbs increases resistance increases power dissipation (brightness) of bulbs increases
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Conclusion <ul><li>Ohmic resistors obey Ohm’s Law linearly </li></ul><ul><li>Resistance is affected by temperature. The resistance of a conductor increases as its temperature increases. </li></ul><ul><li>Light bulbs do not obey Ohm’s Law linearly </li></ul><ul><ul><li>As their temperature increases, the power dissipated by the bulb increases </li></ul></ul><ul><ul><ul><li>i.e. They are brighter when they are hotter </li></ul></ul></ul>
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You’re turn to do some experiments! <ul><li>Now you get to try some experiments of your own, but first, a quick tutorial on the equipment you will be using </li></ul>
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The equipment you’ll be using: <ul><li>- Voltmeter </li></ul><ul><li>- Breadboard </li></ul><ul><li>- Resistors </li></ul><ul><li>- 9V battery </li></ul><ul><li>Let’s do a quick review… </li></ul>
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How to use a voltmeter: <ul><li>Voltmeter: </li></ul><ul><li>- connect either end of the meter to each side of the resistor </li></ul><ul><li>If you are reading a negative value, you have the probes switched. </li></ul><ul><li>There should be no continuity beeping . If you hear beeping, STOP what you are doing and ask someone for help! </li></ul>
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Measuring Voltage Voltage: Probes connect to either side of the resistor
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Breadboards <ul><li>You encountered breadboards early in the year. Let’s review them: </li></ul>The breadboard How the holes on the top of the board are connected:
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Series Resistors are connected such that the current can only take one path
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Parallel Resistors are connected such that the current can take multiple paths
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Real data <ul><li>In reality, the data we get is not the same as what we get in theory. </li></ul><ul><li>Why? </li></ul><ul><li>Because when we calculate numbers in theory, we are dealing with an ideal system. In reality there are sources of error in every aspect, which make our numbers imperfect. </li></ul>
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